1. Technical Field
The invention generally relates to sound processing systems. More particularly, the invention relates to sound processing systems having multiple outputs.
2. Related Art
Audio or sound system designs involve the consideration of many different factors. The position and number of speakers, the frequency response of each speaker, and other factors usually are considered in the design. Some factors may be more pronounced in the design than others in various applications such as a vehicle. For example, the desired frequency response of a speaker located on an instrument panel in a vehicle usually is different from the desired frequency response of a speaker located in the lower portion of a rear door panel. Other factors also may be more pronounced.
Consumer expectations of sound quality are increasing. In some applications, such as a vehicle, consumer expectations of sound quality have increased dramatically over the last decade. Consumers now expect high quality sound systems in their vehicles. In addition, the number of potential audio sources has increased. Audio is available from sources such as radio, compact disc (CD), digital video disc (DVD), super audio compact disc (SACD), tape players, and the like. Furthermore, when a vehicle travels, the signal strength and character of received broadcasts, such as FM radio, vary significantly. As the vehicle changes position with respect to the transmitter, strong stereo signals, weak mono signals, and a continuum of signals with strengths and characters in between may be received. Moreover, many vehicle audio systems employ advanced signal processing techniques to customize the listening environment. Some vehicle audio systems incorporate audio or sound processing that is similar to surround sound systems offered in home theater systems.
Many digital sound processing formats support direct encoding and playback of five or more discrete channels. However, most recorded material is provided in traditional two-channel stereo mode. Matrix sound processors synthesize four or more output signals from a pair of input signals—generally left and right. Many systems have five channels—center, left-front, right-front, left-surround, and right-surround. Some systems have seven or more channels—center, left-front, right-side, left-side, right-side, left-rear, and right-rear. Other outputs such as a separate subwoofer channel may be included.
In general, matrix decoders mathematically describe or represent various combinations of input audio signals in a N×2 or other matrix, where N is the number of desired outputs. The matrix usually includes 2N matrix coefficients that define the proportion of the left and/or right input audio signals for a particular output signal. Typically, these surround sound processors can transform M input channels into N output channels using a M×N matrix of coefficients.
Many audio environments, such as the listening environment inside a vehicle, are significantly different from a home theater environment. Most home theater systems are not designed to operate with the added complexities inside of a vehicle. The complexities include non-optimal driver placement, varying background noise, and varying signal characteristics. A vehicle and similar environments are typically more confined than rooms containing home theatre systems. The speakers in a vehicle usually are in closer proximity to the listener. Typically, there is less control over speaker placement in relation to the listener as compared to a home theater or similar environment, where it is relatively easy to place each speaker the same approximate distance from the listeners.
In contrast, it is nearly impossible in a vehicle to place each speaker the same distance from the listeners when one considers the front and rear seating positions and their close proximity to the doors, as well as the kick-panels, dash, pillars, and other interior vehicle surfaces that could contain the speakers. These placement restrictions are problematic considering the short distances available in an automobile for sound to disperse before reaching the listeners. In many applications within a vehicle, noise is a significant variable. Ambient noise in home theatre systems usually remains relatively constant. However, ambient noise levels in a vehicle can change with speed and road conditions. In addition to noise, the received signal strength, such as of an FM broadcast, varies more as an automobile changes location with respect to the transmission source than in the home environment where the receiver is stationary.